Which of the following statements is the first step in the synthesis and secretion of T3 and T4?
Synthesis of thyroglobulin by the follicular cells
Oxidation of iodide by the follicular cells
Coupling of T1 and T2
lodide trapping by the follicular cells
The Correct Answer is D
a) Synthesis of thyroglobulin by the follicular cells: This occurs after iodide has been trapped and transported into the follicular lumen.
b) Oxidation of iodide by the follicular cells: This is a later step; iodide must first be trapped before it can be oxidized into iodine.
c) Coupling of T1 and T2: This is one of the final steps in T₃/T₄ synthesis where iodinated tyrosines are joined.
d) lodide trapping by the follicular cells
Iodide trapping is the first step, where follicular cells actively transport iodide from the bloodstream into the thyroid follicle using sodium-iodide symporters.
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Related Questions
Correct Answer is B
Explanation
a) Hypophysectomized rat treated with thyroxine: Thyroxine suppresses TSH; no goiter forms.
b) Hypophysectomized rat treated with TSH: TSH directly stimulates thyroid tissue growth, leading to goiter even if the pituitary is absent.
c) Thyroidectomized rat treated with PTU: No thyroid = no goiter can form, and PTU inhibits thyroid hormone production, not causing growth in absent tissue.
d) Thyroidectomized rat treated with TSH: Without thyroid tissue, TSH has no target to stimulate.
Correct Answer is A
Explanation
a) Hypothalamus: Oxytocin is synthesized by neurosecretory cells in the hypothalamus, specifically in the paraventricular and supraoptic nuclei. After production, it is transported down the axons of these neurons to the posterior pituitary gland, where it is stored and released into the bloodstream.
b) Somatic Nervous System: The somatic nervous system is part of the peripheral nervous system and controls voluntary skeletal muscle movements. It does not produce hormones and has no role in endocrine secretion.
c) Anterior Pituitary Gland: The anterior pituitary produces several hormones (like ACTH, GH, PRL, TSH, LH, and FSH), but oxytocin is not one of them. It is not produced by the anterior pituitary; instead, oxytocin is made in the hypothalamus and released from the posterior pituitary.
d) Posterior Pituitary Gland: While the posterior pituitary stores and releases oxytocin, it does not produce it. The production occurs in the hypothalamus, and the posterior pituitary acts as a storage and release site.
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